Per their website the max internal fuel is 52,690. Up to three auxiliary fuel tanks with individual capacity of 8,800 can be loaded in the cargo compartment, which brings the maximum fuel capacity to 79,090.
I had the opportunity to chat with an Embraer C390 engineer at a trade show. I've since lost my notes on what he informed me was the fuel burn per hour and minimum landing fuel. But if you assume 10k per hour and 10k landing fuel, the hypothetical best case is about a 50k to 60k offload, assuming a one to two hour flight.
I didn't get an as detailed explanation on how they plan to fit a boom on it, but he did mention the magic included some flexible and collapsible hoses and couplings.
The 50k to 60k offload and boom sacrifice any cargo capability.
My takeaway was this could be useful as a companion to a small fighter detachment. Remove the internal fuel tanks, leave room for six pallets and 36 maintenance troops. One pallet for bags, one pallet for boom equipment, four pallets for MX equipment. Land somewhere, reconfigure the cargo door to install the boom, have about a 30k offload capability, assuming one hour flight and landing gas (50k internal, 10k flight time, 10k land).
Is the juice worth the squeeze?